During the course of its motion, any aerodynamic profile of a vehicle is exposed to the wake of other profiles of this vehicle, or to phenomena that disturb its boundary layer of air. Aircraft in which the propulsion unit is situated on a pylon are particularly affected because, whatever its design, the pylon generates a wake. This is notably caused by the fact that the thickness of the boundary layer of the profile of the pylon increases in the downstream direction of the profile. Thus, a “velocity shortfall” (or “velocity deficit”) embodied by a difference between the velocity of the free flow of the air and the local velocity of the air in the downstream zone of the profile occurs at the trailing edge of the pylon. The zone exhibiting this velocity shortfall is also the site of a “mass flow rate shortfall” (or “mass flow rate deficit”) regarding the air. As a result, the air has a tendency to be pulled into the velocity shortfall zone, thus causing turbulence.
The discontinuity in the velocities and the turbulence in the wake cause, amongst other things, an increase in the noise generated by the fans of the turbine of the propulsion unit, which may detract from passenger comfort and cause environmental disturbances when the fans pass through the wake of the pylon. This is referred to as a “masking” effect.
There is therefore a need to limit this “masking” effect that gives rise to a variation in pressure in the wake of the pylon. In the specific case of pylons supporting propulsion units, there is a need to eliminate the air flow deficit and therefore to reduce the velocity deficit over the surface thereof.
One of the solutions to this is to blow air from a high-pressure source near the trailing edge of the profile in order to eliminate the air flow deficit and therefore reduce the velocity deficit. To this end, document U.S. Pat. No. 4,917,336 describes an air ejection device comprising an ejection nozzle delivering air, in which device the air escapes through slots made on the suction face and the pressure face of a pylon supporting an aircraft propulsion unit. This solution has the disadvantage of not allowing the openings made in the pylon to be obstructed. A permanent opening on the aerodynamic profile of the pylon at the trailing edge thereof constitutes a break in the aerodynamic profile. Such a break generates disturbances in the air flow and therefore increases the induced drag during phases of flight for which the ejection of air is not needed. Furthermore, with no obturation of the opening, air has a tendency to enter the opening, further disturbing the flow.
In order to solve this problem, document FR 2971765 proposes a similar ejection device further comprising two gratings comprising holes through which air is ejected. The gratings can move relative to one another, allowing the holes to be partially or fully obstructed as the situation dictates, for example as the incidence of the pylon varies. Such a device is, however, complicated to use because it requires a system controlling the opening and closing of the gratings. It is difficult to achieve in terms of manufacturing tolerances and is also not very robust.